Mass spectrometer



Dec. 18, 1956 w. c. WILEY 2,774,881

MASS SPECTROMETER Filed Jan. 25. 1954 4 TTOKNE V United States Patent MASS SPECTROMETER William C. Wiley, Detroit, Mich., assignor to Bendix Aviation Corporation, Detroit, Mich., a corporation of Delaware This invention relates to mass spectrometers and more particularly to mass spectrometers for providing an enhanced resolutionbetween ions of ditterent mass.

There are in present use certain types of mass spectrometerswhich operate to produce a plurality of ions from the molecules of the different gases and vapors in an unknown mixture. When a relatively large number of ions have been produced in the mass spectrometer, a force is imposed upon the ions to accelerate the ions in a pulse from their region of formation. Since the acceleration imparted to the ions of light mass will be greater than the acceleration imparted to the ions of heavy mass, the ions of light mass attain a higher velocity than the ions of heavy mass at the time of removal of the force. Because of this difference in velocity, the ions of light mass travel through a particular distance before the ions of heavy mass. By measuring the different times required for the ions to travel through the particular distance, the masses of the difierent gases and vapors in the unknown mixture can be determined.

To obtain optimum indications in such mass spectrometers, it would be necessary for all the ions of each mass to reach a detector at substantially the same instant of time. This does not always occur in mass spectrometers now in use. One reason is that the ions of a particular mass are located at diiferent positions within their region of retention before a force is applied to accelerate the ions towards a detector. This causes individual ions of the same mass to attain slightly different velocities and to reach the detector at slightly difierent times. The ions positioned at a greater distance from the detector attain a higher velocity because they receive a greater amount of energy from the accelerating force.

It has been found that in mass spectrometers now in use the position of optimum focussing is relatively close to the region of retention of the ions. A detector placed at the position of optimum focussing will detect ions of a particular mass at substantially the same time. However, since this position is relatively close to the region of retention of the ions, the ions of difierent mass do not travel through a sufiicient distance to provide an adequate separation between the ions of difierent mass. As a result the ions of diiferent mass reach the detector within a relatively short period after one another and the measurements become considerably clouded. In the past, attempts have been made to extend the position of optimum focussing to provide for an enhanced resolution between ions of difierent mass. Such attempts have been largely but not entirely successful.

This invention provides amass spectrometer for producing an enhanced resolution between ions of different mass by extending the position of optimum focussing of the ions. This is accomplished by initially providing a field of substantially zero intensity in a first region and a field of a particular intensity in a second region. These fields are subsequently adjusted to provide fields of uniform intensity in the first and second regions.

The fields of uniform intensity in the first and second Patented Dec. 18, 1956 ICC regions areso produced that they attain optimum values instantaneously and remain substantially constant at these optimum values until removal of the fields. In this way, an extended region of uniform intensity is provided and the position of optimum focussing of the ions is correspondingly extended to increase the separation between ions of different mass. Furthermore, since the field provided reaches an optimum value instantaneously and remains substantially constant at this optimum value, the energy imparted to individual ions of a particular mass can be better controlled. Because of these factors, a relatively sharp resolution is provided between the ions of difierent mass.

An object of this invention is to provide a mass spectrometer for distinguishing between ions of different mass by measuring the time required for the ions to travel through a particular distance.

Another object of the invention is to provide a mass spectrometer of the above character for obtaining an enhanced resolution between ions of difierent mass in comparison with the resolution obtained in mass spectrometers now in use.

A further object is to provide a mass spectrometer of the above character for obtaining an improved resolution between ions of different mass by providing a field of uniform intensity through an extended region to increase the distance of optimum focussing of the ions.

Still another object is to provide a mass spectrometer of the above character for initially maintaining the ions in a substantially field-free region and subsequently subjecting the ions to a uniform and extended field of considerable intensity.

A still further object is to provide a mass spectromete of the above character which utilizes a plurality of electrodes and applies particular voltages to the electrodes for producinga field of substantially uniform intensity which reaches an optimum value instantaneously and remains substantially constant at this optimum value.

Another object is to provide a mass spectrometer, of the above character which employs relatively simple electrical circuits for providing a sharp delineation between ions of different mass.

A further object is to provide methods of producing a relatively sharp separation between ions of diflferent mass and of determining the masses of different ions.

Other objects and advantages will be apparent from a detailed description of the invention and from the appended drawings and claims.

The. single figure is a somewhat schematic view, partly in block form and partly in perspective, illustrating one embodiment of the invention. I

In one embodiment of the invention, a suitable wedgeshaped filament 10 made from a suitable material such as tungsten ,is adapted to emit electrons when heated. An electrode 12 is disposed at a relatively short distance such as millimeter from the tip of the filament 10. The electrode 12 is provided with a vertical slot 14, the median position of which is at substantially the same horizontal level as the filament 10.

An electrode 16 is disposed in substantially parallel relationship to the electrode 12 and at a relatively short distance such as 2 millimeters from the electrode 12. The electrode 16 has a slot 18 corresponding substantially in shape and disposition to the slot 14 in the electrode 12. A collector 20 is disposed at a relatively great distance such as 4 centimeters from the electrode 16 and in substantially parallel relationship to the electrode.

A backing plate 22 is positioned between the electrode 16 and the collector 20 in substantially perpendicular relationship to these members. The backing plate 22 is positioned at a relatively short distance such' as /2 centimeter to the rear of an imaginary line extending from the tip of the filament through the slots 14 and 18 to the collector 20. A shield electrode 24 made from a. suitable wire mesh is disposed in substantially parallel relationship to the; backing plate 22 at a relatively short distance such as 1 centimeter from the backing plate. Because of the space between the backing plate 22 and the shield electrode 24, the electrode is a relatively short distance, such as /2 centimeter in front of the imaginary line, disclosed above.

A pair of slatsv 26 made from a suitable insulating material extend from the top and bottom of the backing plate 22.and are suitably connected to the electrode 24 to form a compartment with these members. A horizontal slot 28. is provided in the bottom slat 26, at a position directly below the imaginary line disclosed above. A conduit 30 is connected at one end with the slot 28 and at the other end with a receptacle 32 which is adapted to, hold the, molecules of the different gases-and vapors in an. unknown mixture.

An electrode 34 is positioned in substantially parallel relationship to the electrode 24 at a moderate distance such as 3 centimeters from the electrode. The electrode 34 is provided with a horizontal slot 36. A suitable detector such as a collector 38 is positioned at a relatively great distance such as 7 centimeters from the electrode 3.4. The distance between the collector 38 and. the elec trode 34 should be approximately twice the distance between the imaginary line and the electrode 34 for reasons which will be disclosed in detail hereinafter. An indicator such as an oscilloscope 40 is connected to the collector 38 to indicate the different times. at which ions of dilferent mass reach the collector.

A direct voltage of positive polarity is applied to the electrode 12 through a resistance 42 from a suitable power supply 44. Positive voltages are applied to the collectors 20 and 38 through resistances 46 and 48, respectively, from the power supply 44. These voltages are applied to the collectors 20 and 38 so that the collectors will attract back to them electrons which are secondarily emitted from them upon the impingement of charged particles.

The backing plate 22 and the electrode 24 also have direct voltages of positive polarity applied to them from the power supply 44 through suitable resistances 50 and 52, respectively. For reasons which will be disclosed in detail hereinafter, the voltages applied to the backing plate 22 and the shield electrode 24 are of substantially equal and of relatively large magnitude. For example, the backing plate 22 and the electrode 24 may receive +400 volts from the power supply 44. It has been found that better results are obtained when the voltage on the electrode 24 is slightly higher than the voltage on the backing plate 22. For example, the electrode 24 may be biased at a value of +405 volts. The filament 10 is connected to a grounded resistance 54 and the electrodes 16 and 34 are directly grounded.

The filament 10 and the electrode 12 are connected through coupling capacitances 56 and 58, respectively, to a suitable pulse forming circuit 60 to simultaneously receive voltage pulses having a negative polarity and substantially equal magnitude. A negative voltage pulse is also applied to the shield electrode 24 through a suitable coupling capacitance 62 from the pulse forming circuit 60. This voltage pulse is applied to the electrode 24 a relatively short time, such as a variable period up to 2 microseconds, after the discontinuance of the negative voltage pulses on the filament 10 and the electrode 12. A voltage pulse is also applied to the oscilloscope 40 to initiate the horizontal sweep of the oscilloscope at substantially the. same time as the voltage pulse is applied to the electrode 24. The voltage pulse is applied to the oscilloscope 40 through a coupling capacitance 64 from the pulse forming circuit 60.

Although the pulse forming circuit 60 is shown in block form, its construction and operation are known to persons skilled in the art. For example, Model 902 of the Double Pulse Generator manufactured by the Berkeley Scientific Instrument Company of Richmond, California, may be used to produce a plurality of pulses separated from one another by variable periods of time. This model generator is fully described in a publication entitled Instruction Manual, Berkeley Double Pulse Generator, Model 902 issued by the Berkeley Scientific Instrument Company in, August 1950. The pulse form-- ing circuit disclosed in co-pending application Serial No. 288,014, filed May 16, 1952, by Macon H. Miller and William C. Wiley can also be conveniently adapted for use.

When the filament 10 is heated, a plurality of electrons are emitted by the filament. Because of the positive potential on the electrode 12 with respect to the potential on the filament 10. the electrons are attracted'to- Wards the electrode. Upon passing the slot 14: in the electrode 12, the electrons become decelerated in the region between the electrodes 12 and 16, since the electrode 16 is at a negative potential with respect to the electrode 12. Because the electrons are slowed down in the region between the electrodes 12 and 16, electrons are prevented fromv travelling-into the region between the packing plate 22 and the electrode 24 to ionize the molecules of the difierent gases. and vapors, introduced into the region from the receptacle 32;.

Upon the application of the negative voltage pulsesto the filament 10 and the electrode 12 from the pulse forming circuit 60, the voltage on the electrode 12 becomes negative with respect to the potential of the electrode 16. This difference in potentialcauses the electronspassing through the slot 14 to receive an additional increment in energy in the region between the electrodes 12 and 16. As a result, the electrons travel through the region between the backing plate 22 and: the electrode 24 with sufficient energy toionize themolecules of the difierent gases and vapors introduced into the region. Most of the ions. which are produced have a unitary positive charge.

Since the ions have a positive charge, they are attracted to and retained in the electron stream, which has an opposite charge to that of the ions. Because of the collimating effect produced by the slots 14 and 18 on the electron stream, the ions are retained in a region ofrelatively narrow width. Such a collimating action may also be implemented by a magnetic field (notshown). The pulse forming circuit 60 is set to apply the pulses to the filament 10 and the electrode 12 for a particular time to allow a considerable number of ions to, be produced for retention in the stream. When the pulses are cut off, the ions are available for easy withdrawal upon the application of a voltage pulse to the shield electrode 24 from the pulse forming circuit 60. The pulse forming circuit 60 is set to apply the pulse to the shield electrode 24 at a particular time after the pulse is applied. to the filament 10 and the electrode 12 is cut off,

As previously disclosed, the voltagesnormally applied to the backing plate 22' and the shield electrode 24 are of a substantially equal and relatively large magnitude such as +400 volts. Since the backing plate 22 and the electrode 24 are biased' with substantially equal voltages, an electrical field of' substantially zero intensity is produced between these members. Therefore, no force is imposed upon the ions to withdraw them into the region between the electrode 24 and the electrode 34. As a result, the ions are retained in the region between the backing plate 22 and the electrode 24. If the electrode 24 is maintained at a slightly higher potential than. the backing plate 22 such asa potential of +405 volts,- a slight force is imposed .upon the ions to more effectively prevent the ions from movingpast. the shield electrode 24. Inthe region. between the electrodes 24- and 34 an electrical field of considerable intensity is provided since a difference of 400 volts exists between these members.

Upon the application of a particular voltage pulse to the electrode 24 from the pulse forming circuit 60, an

electrical field of a particular intensity is produced between the backing plate 22 and the electrode 24 and an electrical field of substantially equal intensity is produced between the electrodes 24 and 34. For example, when a voltage pulse having a negative polarity and a magnitude of substantially 100 volts is applied to the electrode 24 the electrode voltage will be reduced to a magnitude of +300 volts. In this way, the difference between the backing plate 22 and the electrode 24 will be 100 volts and the difference between the electrodes 24 and 34 will be 300 volts. Since the distance between the backing plate 22 and the electrode 24 is one centimeter, the intensity of the field between these members will be 100 volts per centimeter. The electrical field between the electrodes 24 and 34 will also be 100 volts per centimeter because of the 300 volt difference between these members, which are separated by a distance of 3 centimeters. The voltage pulse is applied to the electrode 24 for a time sufficient to produce a movement of all the ions past the electrode 24 and the slot 36 in the electrode 34 into the region between the electrode 34 and the collector 38.

Since the electrical field imposed on the ions continues until a movement of the ions into the region between the electrode 34 and the collector 38, the individual ions of a particular mass receive differences in energy dependent upon their initial positioning. Thus, the ions positioned to the rear of the electron stream receive a greater amount of energy than the ions positioned in the front of the electron stream. As a result, individual ions of a given mass will have attained slightly different velocities upon passing through the slot 36 in the electrode 34. In this way, individual ions of a given mass may arrive at the collector 38 before other ions of the same mass to cloud the measurements which are obtained in mass spectrometers now in use.

Errors resulting from the differences in the initial positioning of the ions can be minimized if the collector is placed at a focal distance from the electrode 34 to permit the ions in the rear of the electron stream to overtake the ions in the front of the electron stream. This distance has been found by analysis to be substantially twice the distance between the imaginary line disclosed above and the electrode 34, as fully disclosed in co-pending application Serial Number 251,352, filed October 15, 1951, by William C. Wiley and William H. Wells. By placing the collector 38 at this distance from the electrode 34, optimum focussing is provided so that individual ions of a given mass will reach the collector at substantially the same time.

Certain types of mass spectrometers now in use employ a single electrode ion source which includes members similar to the backing plate 22 and the electrode 34. With such an ion source, the electrode must be positioned relatively close to the backing plate in order to provide a field of uniform intensity throughout the region between the backing plate and the electrode. Because of this, the imaginary line disclosed above or the path of the electron stream is relatively close to the electrode. Since the position of optimum focussing is twice this distance, a collector placed at the position of optimum focussing would be relatively close to the electrode. Therefore, the ions travel a relatively short distance before they reach the collector and the ions do not have adequate time to become materially separated on the basis of their mass. This causes the measurements obtained to be somewhat clouded since the ions of different mass will reach the collector within relatively short periods after one another. This is true even though the ions of each particular mass will impinge upon the collector at substantially the same time.

This invention provides a mass spectrometer in which the position of optimum focussing is extended to produce an enhanced resolution between ions of different mass. The position of optimum focussing is extended by disposing the shield electrode 24 between the backing plate 22 and the electrode 34. In this way, the region through which an electrical field of uniform intensity can be provided is considerably increased. As a result, the electrode 34 may be positioned at a greater distance from the backing plate 22 than has been possible in mass spectrometers now in use. Since the electrode 34 is at a greater distance from the backing plate 22, the distance from the electrode to the imaginary line disclosed above is also greater. Since the position of optimum focussing is substantially twice the distance from the imaginary line to the electrode 34, the collector 38 may be positioned at a greater distance from the electrode. This causes the ions to travel through a greater distance before they are detected. Therefore the ions have adequate time to become materially separated on the basis of their massto provide for an enhanced resolution between ions of different mass.

The mass spectrometer disclosed above has several important advantages. By the imposition of voltages of substantially equal and of relatively great magnitude on the backing plate 22 and the shield electrode 24 during the period that the ions are being formed, a substantially field-free region is produced between the plate and the electrode. Because of this field-free region between the backing plate 22 and the shield electrode 24, substantially no force is imposed on the ions to withdraw them from the electron stream and the ions are retained in a region having a relatively narrow width until a voltage pulse is applied to the electrode 24. Furthermore, a

slight repelling force may actually be imposed on the ions in the region between the backing plate 22 and the electrode 24 to insure that the ions remain in this region.

Another advantage is that a voltage pulse of only moderate magnitude need be applied to the electrode 24 to produce fields of uniform intensity between the backing plate 22 and the electrode 24 and between the electrodes 24 and 34. For example, a voltage pulse of moderate magnitude such as l00 volts may be applied to the electrode 24. Such a moderate voltage pulse can be provided with a relatively fast rise time and with a flat top to provide a substantially rectangular pulse for application to the electrode 24. A rectangular pulse is advantageous in that the desired electrical field is instantaneously produced and is thereafter maintained at a constant level until all the ions have moved past the electrode 34. In this way, optimum focussing is obtained for individual ions of each particular mass.

Since the backing plate 22 is maintained at a relatively high potential and fields of uniform intensity are provided by the imposition of a voltage pulse of moderate magnitude to the electrode 24, there is provided between the backing plate 22 and the electrode 34 a single region having a uniform field of considerable intensity. For example, if the voltage applied to the backing plate 22 is +400 volts, a uniform field of volts per centimeter will be produced between the backing plate and the electrode 34 upon the application of a voltage pulse of only moderate magnitude to the shield electrode 24.

Another advantage is that only a single pulse is re quired to produce an electrical field of uniform intensity through a region of extended length. It will be recognized that such a uniform field may be produced in other ways. For example, a single pulse of moderate magnitude and of positive polarity may be applied to the backing plate to produce a similar result.

It will also be recognized that by changing the position of the electrode 24, the magnitude of the voltage pulses applied to the electrode may be varied. For example, if the electrode 24 is positioned closer to the backing plate 22, the voltage pulse applied to the electrode 24 may be correspondingly reduced to provide an electrical field of uniform intensity. Because the mass spectrometer requires only a single voltage pulse of moderate magnitude to produce an enhanced resolution between ions of difiEerent mass, the mass spectrometer may be constructed with fewer components and with simpler circuitry than has been heretofore employed in mass spectrometers.

In order for an optimum resolution to be obtained between ions of difierent mass, the electrical field produced between the backing plate 22 and the electrode 34 has to be substantially constant. Since the backing plate 22 and the electrode 36 are separated by approxi mately 4 centimeters, the lateral and vertical dimensions of these members would have to be relatively great in order to maintain a substantially constant electrical field between them. By utilizing the shield electrode 24 to reinforce the electrical field, the lateral and vertical dimensions of the backing plate 22 and the electrode 36 can be considerably reduced.

Although this invention has been disclosed and illustrated with reference to particular applications, the principles involved are susceptible of numerous other applications which will be apparent to persons skilled in the art. The invention is, therefore, to be limited only as indicated by the scope of the appended claims.

What is claimed is:

l. A mass spectrometer, including, means for providing a first region, means for providing a second region past the first region, means for providing a plurality of ions in the first region, means for initially applying a force of substantially Zero magnitude in the first region to retain the ions in the region and for applying a force of considerable magnitude in the second region, means for adjusting the forces applied in the first and second regions to provide in the regions forces of uniform strength to accelerate individual ions of each particular mass through the regions at a substantially uniform rate, means disposed past the second region at a position of optimum focus for detecting the ions, and means for indicating the relative times at which the ions are detected.

2. A mass spectrometer, including, means for providing a first region, means for providing a second region adjacent to the first region, means for providing a plurality of ions in the first region, first electrical circuitry for initially imposing an electrical field in the first region to restrain the ions from movement out of the region and for imposing a field of particular intensity in the second region, second electrical circuitry for adjusting the fields in the first and second regions to provide fields of substantially uniform intensity in the regions for producing a movement of the ions through the regions and for imparting slightly greater velocities to individual ions of a particular mass initially positioned to the rear of the first regon than to ions of the same mass positioned in the front of the first region, means disposed at a position of optimum focussing of the ions to detect the ions of each mass at substantially the same instant of time, and means for indicating the relative times at which ions of different mass are detected.

3. A mass spectrometer, includin a first electrode, a second electrode disposed at a particular distance past the first electrode, a third electrode disposed at a par ticular distance past the second electrode, means for providing a plurality of ions in the region between the first and second electrodes, an electrical circuit for providing an electric field between the first and second electrodes to retain the ions in the region between the electrodes and for providing an electric field of considerable intensity between the second and third electrodes, an electrical circuit for adjusting the field between the first and second electrodes to provide a field of substantially uniform intensity between the first and third electrodes for producing a movement of the ions towards thethird electrode at a substantially uniform rate of acceleration for the ions of each particular mass and for producing a separation of ions on the basis of their-mass, means disposed at the position of optimum focussing of the ions to detect the ions, and means for indicating the relative times at which the ions are detected.

4. A mass spectrometer, including, a first electrode, a second electrode disposed at a particular distance from the first electrode to provide a first region between the electrodes, a third electrode disposed at a particular distance from the second electrode to provide a second region between the electrodes, means for providing a plurality of ions in the first region, an electrical circuit for applying voltages of substantially equal magnitude to the first and second electrodes to provide a field of substantially zero intensity in the first region for the retention of the ions in the region and to provide a field of particular intensity in the second region, an electrical circuit for applying voltage pulses to the second electrode relative to the voltage on the first electrode to provide fields of substantially equal intensity in the first and second regions for producing a movement of the ions through the regions, the voltage pulses being applied until a movement of the ionsthrough the second region, means disposed at the position of optimum iocussing of the ions to detect the ions of each mass at substantially the same instant of time, and means for indicating the relative times at which therions of different mass are detected.

5. A mass. spectrometer, including, a first electrode, a second electrode disposed at a particular distance from thefirst electrode to provide afirst region between the electrodes, a thirdpelectrode disposed at a particular distance from the .second electrode to provide a second region between the electrodes, means for providing a plurality of ions in the first region, an electrical circuit for initially applying voltages of substantially equal magnitude to the first and second. electrodes to produce an electric field in the first region for preventingiany movementof the ions through the region .and to provide in the second region an electric field having a particular intensity, .an electrical circuit for applying voltage pulses of relatively moderate magnitude between the first and second electrodes for providingvelectric fields of substantially uniform intensity in the first and second regions to produce a movement-of the ion through the regions and to impart a substantially constant rate of acceleration to the ions of each mass, means disposed past the third electrode at substantially the position of optimum focusing of the ions, and means for indicatingithe relative timesv at which the ion are detected.

6. A mass spectrometer, including, a backing plate, a first electrode disposed at a particular distance from thebacking plate, a second electrode disposed at a particular distance from the first electrode, means for applying voltages to the backing plate and the first electrode to provide in the region between the plate and the electrode a field of relatively moderate intensity for retaining the 1 ions within the region and to provide a field of considerable intensity in the region between the first and second electrodes, means for providing a plurality of ions in the region between the first and second electrodes for the retention of the ions in the region, means for adjusting the voltage on the first electrode relative to the voltage on the backing plate to provide fields of substantially equal intensity between the backing plate and the first electrode and between the first and second electrodes for producing a movement of 'theions past the first and second electrodes and for imparting a slightly greater amount of energy to ions of a given mass initially positioned closer to the backing plate than to ions of the same mass initially positioned closer to the first electrode, means disposed at substantially the position of optimum focussing of the ions to detect the ions of each particular mass at substantially the same time, and means for indicating the relative times at which the ions of different mass are detected.

7. A mass spectrometer, including, a backing plate, a first electrode disposed at a particular distance from the backing plate, a second electrode disposed at a particular distance from the first electrode, an electrical circuit for imposing voltages on the backing plate and the first electrode to provide an electrical field for restraining the movement of ions towards the electrode and to provide a field of considerable intensity between the first and second electrodes, means for providing a plurality of ions in the region between the backing plate and the first electrode for the retention of the ions in the region, an electrical circuit for imposing voltage pulses of relatively moderate magnitude between the backing plate and the first electrode to provide an electric field of substantially uniform and relatively great intensity between the backing plate and the second electrode for producing a movement of the ions past the first and second electrodes and for separating the ions on the basis of their mass, mean disposed at a particular distance from the second electrode to detect the ions of any given mass at substantially the same time, and means for indicating the relative times at which ions of different mass are detected.

8. A mass spectrometer, including, a first electrode, a second electrode disposed at a particular distance from the first electrode, a third electrode disposed .at a particular distance from the second electrode, an electrical circuit for imposing voltages on the first and second electrodes to restrain any movement of ions towards the second electrode and voltages of considerable intensity on the first and second electrodes relative to the voltage on the third electrode, means for providing a plurality of ions in the region between the first and second electrodes, an electrical circuit for applying voltage pulses of relatively moderate magnitude between the first and second electrodes to produce between the first and second electrodes and between the second and third electrodes fields of substantially uniform intensity for a movement of the ions past the second and third electrodes and for a separation of the ions on the basis of their mass, means disposed at substantially the position of optimum focussing of the ions to detect the ions of each mass at substantially the same time, and means for indicating the diiferent times required to detect the ions of each mass.

9. A mass spectrometer, including, a first electrode, a second electrode disposed at a particular distance from the first electrode, a third electrode disposed at a particular distance from the second electrode, mean for applying voltages to the first and second electrodes to provide between the electrodes an electrical field for preventing the movement of any ions towards the second electrode and to provide between the second and third electrodes an electrical field of particular intensity, means for providing a plurality of ions between the first and second electrodes in a region of relatively restricted width, means for applying a voltage pulse between the first and second electrodes to produce between the first and second electrodes and between the second and the third electrodes electrical fields of substantially uniform intensity for producing a movement of the ions past the second and third electrodes and for producing a separation of the ions on the basis of their mass, means disposed at a distance from the third electrode equal to substantially twice the distance between the region of provision of the ions and the third electrode to detect the ions of each mass at substantially the same instant of time, and means for indicating the relative times at which ions of different mass are detected.

10. A mass spectrometer, including, a first electrode, a second electrode positioned at a particular distance from the first electrode, a third electrode positioned at a particular distance from the second electrode, means for providing a plurality of ions between the first and second electrodes and for retaining the ions in a region of relatively narrow width, an electrical circuit for imposing voltages on the first and second electrodes to provide between the electrodes an electrical field for restraining the movement of the ions from their region of retention and to provide between the second and third electrodes an electrical field of particular intensity, an electrical circuit for imposing a voltage pulse between the first and second electrodes to provide between the first and third electrodes an electrical field of substantially uniform intensity to produce a movement of the ions past the second and third electrodes and to produce a separation of the ions on the basis of their mass, a detector disposed at a distance from the third electrode equal to substantially twice the dis tance between the region of retention of the ions and the third electrode, and means for indicating the relatives times at which ions of diiferent mass are detected.

11. A mass spectrometer, including, a first electrode, a second electrode disposed a particular distance from the first electrode, means for biasing the first and second electrodes relative to one another to provide between them an electric field of uniform intensity for the acceleration of ions, a shield electrode disposed between the first and second electrodes, means for biasing the shield electrode relative to the first electrode to disrupt the uniform electric field between the first and second electrodes and to prevent any ions provided between the first and shield electrodes from moving towards the second electrode, means for providing a plurality of ions between the first and shield electrodes, means for applying a voltage pulse to the shield electrode to restore the electric field of uniform intensity between the first and second electrodes to produce an acceleration of the ions through the region between them, and means disposed a particular distance from the second electrode to detect the ions.

References Cited in the file ofthis patent UNITED STATES PATENTS 2,685,035 Wiley July 27, 1954 OTHER REFERENCES A Pulsed Mass Spectrometer with Time Dispersion, by Wolff et al., published in The Review of Scientific Instruments, vol. 24, No. 8, August 1954, pages 616, 617. 

